Tunneling barrier infrared detector devices

ABSTRACT

Embodiments of the present disclosure are directed to infrared detector devices incorporating a tunneling structure. In one embodiment, an infrared detector device includes a first contact layer, an absorber layer adjacent to the first contact layer, and a tunneling structure including a barrier layer adjacent to the absorber layer and a second contact layer adjacent to the barrier layer. The barrier layer has a tailored valence band offset such that a valence band offset of the barrier layer at the interface between the absorber layer and the barrier layer is substantially aligned with the valence band offset of the absorber layer, and the valence band offset of the barrier layer at the interface between the barrier layer and the second contact layer is above a conduction band offset of the second contact layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/979,745, filed on Apr. 15, 2014, the contents of which areincorporated by reference in their entirety

BACKGROUND

Field

The present specification generally relates to infrared detector devicesand, more particularly, to tunneling barrier infrared detector devicesand structures for reducing dark current, lowering bias voltage, andincreasing operating temperature for infrared detectors such as focalplane arrays.

Technical Background

The nBn device structure has been used to improve the operatingtemperature of photoconductive infrared detectors by blocking the flowof electrons. The nBn device structure generally includes an n-typeabsorber layer, a barrier layer to block majority carriers, and ann-type contact layer. Such nBn devices can be used to improve theoperating temperature of an infrared focal plane arrays (FPA). FPAdevices using the nBn device structure require significant bias to turnon the photocurrent, while the dark current density increases at thesame time. To increase the operating temperature further, it isnecessary to further reduce the dark current and lower the bias voltagerequired to turn on the photocurrent. If a zero bias photoresponse canbe achieved, it will provide more room for FPA biasing optimizations andthus better imaging quality.

Accordingly, a need exists for alternative infrared detector deviceswith lower dark current, lower required bias voltages, and increasedoperating temperature.

SUMMARY

These and additional features provided by the embodiments describedherein will be more fully understood in view of the following detaileddescription, in conjunction with the drawings.

Embodiments described herein are directed to tunneling barrier infrareddetector (“TBIRD”) structures. In some embodiments, a TBIRD architectureallows minority carrier (holes) to tunnel through the barrier structureinto the conduction band of the contacting layer and get collected. Inother embodiments, a Fast TBIRD architecture is particularly suited forp-type absorption materials where the minority electrons can tunnelthrough the “barrier” layers in the device interior, while on thesurface, due to the natural oxidization of the “barrier” layers, currentflow is blocked. This resolves the passivation issue present in devicesutilizing p-type absorbers, where exposed sidewall surface is invertedto n-type.

The TBIRD structures described herein provide a universal solution forinfrared detectors with a cutoff wavelength ranging from short-waveinfrared (“SWIR”) to very long-wave infrared (“VLWIR”) and beyond.

In one embodiment, an infrared detector device includes a first contactlayer, an absorber layer adjacent to the first contact layer, and atunneling structure including a barrier layer adjacent to the absorberlayer and a second contact layer adjacent to the barrier layer. Thebarrier layer has a tailored valence band offset such that a valenceband offset of the barrier layer at the interface between the absorberlayer and the barrier layer is substantially aligned with the valenceband offset of the absorber layer, and the valence band offset of thebarrier layer at the interface between the barrier layer and the secondcontact layer is above a conduction band offset of the second contactlayer.

In another embodiment, an infrared detector device includes a firstcontact layer, an absorber layer adjacent to the first contact layer,wherein the absorber layer is doped p-type, a first barrier layeradjacent to the absorber layer, an n-type layer adjacent to the firstbarrier layer, a second barrier layer adjacent to the n-type layer, anda second contact layer adjacent to the second barrier layer. A bandgapof the n-type layer is greater than a bandgap of the absorber layer. Avalence band offset of the first barrier layer at an interface betweenthe first barrier layer and the n-type layer is above a conduction bandoffset of the n-type layer.

In yet another embodiment, an infrared detector device includes a firstcontact layer, an absorber layer adjacent to the first contact layer,wherein the absorber layer is doped p-type, a hole barrier layeradjacent to the absorber layer, a first barrier layer adjacent to thehole barrier layer, a second barrier layer adjacent to the first barrierlayer, and a second contact layer adjacent to the second barrier layer.A bandgap of the hole barrier layer is greater than a bandgap of theabsorber layer. The hole barrier layer includes a p-n junction. Avalence band offset of the first barrier layer is above a conductionband offset of the hole barrier layer at an interface between the holebarrier layer and the first barrier layer.

In yet another embodiment, an infrared detector device includes a firstcontact layer, an absorber layer adjacent to the first contact layer,wherein the absorber layer is doped p-type, a hole barrier layeradjacent to the absorber layer, an n++ layer adjacent the hole barrierlayer, wherein a thickness of the n++ layer is less than a thickness ofthe hole barrier layer, a barrier layer adjacent to the n++ layer,wherein the barrier layer is doped p++, and a second contact layeradjacent to the barrier layer, wherein the second contact layer is dopedp+. A bandgap of the hole barrier layer is greater than a bandgap of theabsorber layer. The hole barrier layer includes a p-n junction.

In yet another embodiment, a dual-band infrared detector device includesa first absorber layer, wherein the first absorber layer is capable ofabsorbing radiation in a first wavelength range, a first barrier layeradjacent to the first absorber layer, a second barrier layer adjacent tothe first barrier layer, and a second absorber layer, wherein the secondabsorber layer is capable of absorbing radiation in a second wavelengthrange. Wavelengths within the second wavelength range are shorter thanwavelengths in the first wavelength range. The first barrier layer, thesecond barrier layer, and the second absorber layer form a tunnelingstructure; A valence band offset of the second barrier layer at aninterface between the second barrier layer and the second absorber layeris above a conduction band offset of the second absorber layer. Underzero bias or negative bias wherein a positive potential is present atthe first absorber layer with respect to the second absorber layer, thedual-band infrared detector device provides a long-wave response. Underpositive bias wherein a positive potential is present at the secondabsorber layer with respect to the first absorber layer, the dual-bandinfrared detector device provides a mid-wave response.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments set forth in the drawings are illustrative and exemplaryin nature and not intended to limit the subject matter defined by theclaims. The following detailed description of the illustrativeembodiments can be understood when read in conjunction with thefollowing drawings, wherein like structure is indicated with likereference numerals and in which:

FIG. 1 is a schematic illustration of an exemplary tunneling barrierinfrared detector (“TBIRD”) device according to one or more embodimentsdescribed and illustrated herein;

FIG. 2A is a graphic illustration of a band diagram for an example TBIRDdevice having a triangular tunneling structure according to one or moreembodiments described and illustrated herein;

FIG. 2B is a graphic illustration of a band diagram for an example TBIRDdevice having a parabolic tunneling structure according to one or moreembodiments described and illustrated herein;

FIG. 2C is a graphic illustration of a band diagram for an example TBIRDdevice having a rectangular tunneling structure according to one or moreembodiments described and illustrated herein;

FIG. 3 is a graphic illustration of a band diagram for a simulation ofan example mid-wave infrared TBIRD device having a graded barrier layeraccording to one or more embodiments described and illustrated herein;

FIG. 4 is a graphic illustration of a band diagram for a simulation ofan example long-wave infrared TBIRD device having a graded barrier layeraccording to one or more embodiments described and illustrated herein;

FIG. 5 is a graphic illustration of a band diagram for a simulation ofan example long-wave infrared TBIRD device having a rectangular holewell according to one or more embodiments described and illustratedherein;

FIG. 6 is a graphic illustration of a band diagram for a simulation ofan example very long-wave infrared TBIRD device having a graded barrierlayer according to one or more embodiments described and illustratedherein;

FIG. 7 is a graphic illustration of a band diagram for a simulation ofan example very long-wave infrared TBIRD device having a rectangularhole well according to one or more embodiments described and illustratedherein;

FIG. 8A is a graphic illustration of a band diagram for an examplemid-wave/long-wave dual band device having a tunneling structureaccording to one or more embodiments described and illustrated herein,wherein carrier propagation is illustrated for zero or negative biasdirection;

FIG. 8B is a graphic illustration of a band diagram for the examplemid-wave/long-wave dual band device depicted in FIG. 8A, wherein carrierpropagation is illustrated for positive bias direction;

FIG. 9 is a graphic illustration of a band diagram for an example fasttunneling barrier infrared detector (“Fast TBIRD”) device having a firstbarrier layer and a second barrier layer according to one or moreembodiments described and illustrated herein;

FIG. 10 is a graphic illustration of a band diagram for a simulation ofan example Fast TBIRD device as depicted in FIG. 9 wherein the absorberlayer is an LWIR material according to one or more embodiments describedand illustrated herein;

FIG. 11 is a graphic illustration of a band diagram for an example FastTBIRD device having a first barrier layer and a second barrier layerconfigured as a TBIRD structure according to one or more embodimentsdescribed and illustrated herein;

FIG. 12 is a graphic illustration of a band diagram for a simulation ofan example Fast TBIRD device as depicted in FIG. 11 according to one ormore embodiments described and illustrated herein;

FIG. 13 is a graphic illustration of a band diagram for an example FastTBIRD device having a single electron barrier with a tunneling structureand a wider-gap p-n junction prior to the tunneling junction accordingto one or more embodiments described and illustrated herein;

FIG. 14 is a graphic illustration of a band diagram for a simulation ofan example Fast TBIRD device as depicted in FIG. 13 according to one ormore embodiments described and illustrated herein;

FIG. 15 is a graphic illustration of a band diagram for a simulation ofan example Fast TBIRD device derived from the example Fast TBIRD deviceas depicted in FIG. 13 according to one or more embodiments describedand illustrated herein; and

FIG. 16 is a graphical illustration of a band diagram for a simulationof an example Fast TBIRD device derived from the example Fast TBIRDdevice as depicted in FIG. 13 according to one or more embodimentsdescribed and illustrated herein.

DETAILED DESCRIPTION

Embodiments of the present disclosure are directed to tunneling barrierinfrared detector (“TBIRD”) devices and structures. Such TBIRDstructures may be incorporated into infrared applications, such as focalplane arrays. Embodiments described herein are generically applicable toinfrared detector devices regardless of wavelength regime, e.g.,short-wave infrared (“SWIR”), mid-wave infrared (“MWIR”), long-waveinfrared (“LWIR”), very long-wave infrared (“VLWIR”), far infrared(“FIR”), and the like, to reduce the dark current level and provideclose to zero bias optical turn on.

More specifically, in some embodiments a TBIRD architecture provides atunneling structure that allows minority carrier (holes) to tunnelthrough a barrier layer into the conduction band of the contacting layerand get collected. As described in more detail below, the tunnelingstructure may be provided by a dedicated tunneling layer adjacent to abarrier layer, or may be incorporated into the barrier layer directed byused of a graded material.

In other embodiments, a Fast TBIRD architecture is used in conjunctionwith p-type absorption materials where the minority electrons are ableto tunnel through the one or more barrier layers in the device interior.Current flow is blocked on the surface of the device due to the naturaloxidization of the barrier layers. This resolves the passivation issuepresent in devices utilizing p-type absorbers, especially those usingGa-free superlattice structures (“SLS”).

The TBIRD structures described herein provide a universal solution forinfrared detectors with a cutoff wavelength ranging from SWIR to FIR.

Referring to FIG. 1, an example TBIRD structure 100 is schematicallyillustrated. The example TBIRD structure generally comprises a substrate105, one or more first contact layers 110 (i.e., one or more firstcollector layers) disposed on the substrate 105, an absorber layer 120disposed on the one or more first contact layers 110, one or morebarrier layers 130 disposed on the absorber layer 120, and one or moresecond contact layers 140 (i.e., one or more collector layers) disposedon the one or more barrier layers 130. As described in more detailbelow, the one or more barrier layers 130 and one or more second contactlayers 140 form a tunneling structure 133.

In the example TBIRD structure depicted in FIG. 1, the first contactlayer 110 is a doped n-type semiconductor material, while the secondcontact layer 140 is a doped p-type material. The absorber layer 120 isan n-doped semiconductor material capable of absorbing photons in adesired wavelength range. The absorber layer 120 may be fabricated frommaterials capable of absorbing wavelengths in any infrared wavelengthrange, such as near-infrared, short-wave infrared (“SWIR”), mid-waveinfrared (“MWIR”), long-wave infrared (“LWIR”), very long-wave infrared(“VLWIR”), and far infrared. As non-limiting examples, the absorberlayer 120 may comprise an n-doped InAs/InAsSb SLS, InAs/Ga(In)Sb SLS, adigital alloy (e.g., InAsSb absorber with GaAs strain balancer), or bulkabsorber (e.g., bulk InAsSb), and the barrier layer 130 may compriseAlAsSb, AlGaAsSb, AlSb/InAs SLS, or AlSb/InAs/GaSb SLS etc. In someembodiments, the contact layer 140 is n-doped InAsSb, InAs/InAsSb SLS,or InAs/Ga(In)Sb SLS. The substrate 105 may be fabricated from anysuitable base substrate, such as GaSb. Although examples are given formaterials grown on GaSb substrate, similar device structures may beformed on other types of substrates, such as InAs, InP, GaAs, and thelike.

FIGS. 2A-2C depict band diagrams for various implementations of theTBIRD structure 100 depicted in FIG. 1. Line E_(F) represents the Fermienergy, while line 102 is the conduction band and line 104 is thevalence band of the TBIRD structure 100. FIGS. 2A-2C depict an absorberlayer 120, barrier layers 130 comprising a first barrier layer 131 and asecond barrier layer 132, and a second contact layer 140. It is notedthat the substrate layer and the first contact layer are not depicted inFIGS. 2A-16 for ease of illustration. Although FIGS. 2A-2C illustrate afirst barrier layer 131 and a second barrier layer 132, in otherembodiments only a single barrier layer 130 is provided (e.g., a gradedbarrier layer).

As shown in FIGS. 2A-2C, the barrier layer(s) 130 and the second contactlayer(s) 140 define a tunneling structure 133 that is tailored such thatthe valence band offset (“VBO”) at the interface between the secondbarrier layer 132 and the contact layer 140 is above the conduction bandoffset (“CBO”) 142 of the contact layer 140 to provide a tunnelingjunction interface. Therefore, the base energy level within the holequantum well defined by the tunneling structure 133 is above the CBO 142at the interface between the second barrier layer 132 and the secondcontact layer 140. FIGS. 2A-2C illustrate a tunneling structure 133providing a quantum well for holes having a variety of shapes. Althoughthe shape of the tunneling structure 133 providing the potential wellfor holes at the tunneling junction interface is illustrated as linearor triangular in FIG. 2A, as parabolic in FIG. 2B, or rectangular (i.e.,discrete) as in FIG. 2C, it may take any shape as long as the lowesthole energy level (i.e., the valence band 104 at point 135) is above theCBO 142 of the contact layer 140 (i.e., the conduction band 102 at point142). The VBO 134 of the first barrier layer 131 is substantiallyaligned with the VBO of the absorber layer 120 at the interface betweenthe first barrier layer 131 and the absorber layer 120.

Once an incident photon generates an electron-hole pair in the absorberlayer 120, the hole first diffuses to the barrier layer(s) 130. Due tothe potential drop within the barrier layer 130, the holes drop to thelowest available hole states within the quantum well formed within thebarrier layer(s) 130 (e.g., second barrier layer 132), and recombinewith electrons from the contact layer 140 at the interface between thebarrier layer(s) 130 (e.g., second barrier layer 132) and the contactlayer 140. The charge neutrality requirement ensures that there will beequal number of electrons from contact metal flowing in forcompensation. Thus, the signal is propagated out of the device.

As an example and not a limitation, for an absorber material (e.g.,MWIR, LWIR, or VLWIR (on GaSb substrates)), the contact layer 140material may be InAs_(0.9107)Sb_(0.0893) lattice-matched to GaSb. AsInAs_(0.9107)Sb_(0.0893) has a cutoff wavelength of <4.0 μm below 150K,it would not affect the absorption near the band edge of the absorberlayer 120. The validity of the TBIRD structure is applicable to SLS witharbitrary cutoff from SWIR to VLWIR and is universal. Once the minoritycarrier is collected in the InAsSb contact layer 140, the signal currentflow is carried by electrons, instead of holes. This may make thecollection efficiency very high due to orders of magnitude higherelectron mobility than the holes. In addition, disadvantages of Ohmiccontact to the valence band hole carriers while the material is n-typeis avoided with the proposed tunneling structure.

An example embodiment of a MWIR TBIRD device design illustrated by asimulation band diagram 301 is shown in FIG. 3. The illustratedembodiment uses a MWIR absorber material for layer 120, a graded(GaSb)_(x)(AlAs_(0.0835)Sb_(0.9165))_(1-x) barrier 130 and a secondcontact layer 140 of (GaSb)_(y)(InAs_(0.9107)Sb_(0.0893))_(1-y). Thecomposition x within the barrier layer 130 is graded, or varied withlayer thickness, such that under zero bias, the VBO within the barrieris linearly dependent on the layer thickness. In alternativeembodiments, the barrier layer 130 may be made of a graded barriersuperlattice material as well, such as AlAsSb/GaSb SLS with varyinglayer thickness to achieve better controllability. The contact layer 140may be n-doped in the 10¹⁷˜10¹⁸ cm⁻³ range.

As shown in FIG. 3, the tunneling structure 133 defined by the barrierlayer 130 and the second contact layer 140 is triangular in shape. TheVBO at the interface between the barrier layer 130 and the secondcontact layer 140 is above the CBO, as shown by lines 104 and 102. Thetunneling structure 133 therefore provides a quantum well for holes andallows holes to “tunnel through” the barrier layer 130.

An example embodiment of a LWIR TBIRD device design illustrated by asimulation band diagram 401 is shown in FIG. 4. The absorber layer 120is fabricated from a LWIR material. The barrier layer 130 is fabricatedfrom a material similar to the barrier layer 130 of FIG. 3 such that theVBO of the barrier layer is substantially aligned with VBO of theabsorber layer 120 on at the interface between the absorber layer 120and the barrier layer 130, as shown by line 104. The contact layer 140may be (GaSb)_(x)(InAs_(0.9107)Sb_(0.0893))_(1-x) doped n-type asdescribed above with respect to FIG. 3.

As shown in FIG. 4, the tunneling structure 133 within the barrier layer130 is triangular in shape. The VBO of the barrier layer 130 at theinterface between the barrier layer 130 and the second contact layer 140is above the CBO, as shown by lines 104 and 102. The tunneling structure133 therefore provides a quantum well for holes and allows holes to“tunnel through” the barrier layer 130.

An example embodiment of a LWIR TBIRD device design illustrated by asimulation band diagram 501 is shown in FIG. 5. The structure of thesimulated device is similar to the TBIRD device depicted in FIG. 1,where the barrier layer comprises a first barrier layer 131 and a secondbarrier layer 132. This example device design has a first barrier layer131 fabricated from (GaSb)_(x)(AlAs_(0.08)Sb_(0.92))_(1-x) latticematched to GaSb, with x value chosen such that the VBO of the firstbarrier layer 131 is lined up with that of the absorber layer 120. Inthis embodiment, a second barrier layer 132 fabricated from a thin layerof (GaSb)_(y)(AlAs_(0.08)Sb_(0.92))_(1-y) (y>x) forming a quantum wellfor the holes prior to the contacting collector material of the contactlayer 140. In the illustrated embodiment, the second barrier layer 132is p+ doped to the order of 10¹⁸ cm⁻³ for a well thickness of 300 Å. Thecontact layer 140 is fabricated from InAs_(0.9107)Sb_(0.0893) that islattice matched to GaSb. The contact layer 140 may be doped to achieveproper band alignment.

As a general rule, as used herein, p−/n− doping levels are on the orderof 10⁻¹⁶ cm⁻³ or lower, p/n doping levels are on the order of 10¹⁶˜10¹⁷cm⁻³, p+/n+ doping levels are on the order of 10¹⁸ cm⁻³, and p++/n++doping levels are on the order of 10¹⁹ cm⁻³ or beyond.

In the simulation band diagram 501 depicted in FIG. 5, the tunnelingstructure 133 formed by the second barrier layer 132 and the secondcontact layer 140 is configured as a step-wise transition from a firstVBO of the first barrier layer 131 to a second VBO of the second barrierlayer 132 at the interface between the first barrier layer 131 and thesecond barrier layer 132. The second VBO is above the CBO of the secondcontact layer 140 at the interface between the second barrier layer 132and the contact layer 140, thereby providing the hole tunneling functiondescribed above.

Another example embodiment of a VLWIR TBIRD device design is illustratedby the simulation band diagram 601 of FIG. 6. The absorber layer 120 isfabricated from a VLWIR material. The barrier layer 130 is fabricatedfrom the same type of material as described above with respect to FIG.3. The barrier layer 130 is configured such that the VBO lines up withthat of the absorber layer 120 at the interface between the absorberlayer 120 and the barrier layer 130. The contact layer 140 of theexample device illustrated by FIG. 6 is fabricated from(GaSb)_(0.325)(InAs_(0.91)Sb_(0.09))_(0.675). As shown by lines 102 and104 in FIG. 6, the VBO of the barrier layer 130 is above the CBO at theinterface between the barrier layer 130 and the contact layer 140.

Another example embodiment of a VLWIR TBIRD device design is illustratedby the simulation band diagram 701 of FIG. 7. Instead of a graded thebarrier layer, the barrier layer 130 comprises a first barrier layer 131and a thin second barrier layer 132 of(GaSb)_(y)(AlAs_(0.08)Sb_(0.92))_(1-y) that in part defines a tunnelingstructure 133 forming a quantum well for the holes prior to the contactlayer 140 material. In the illustrated embodiment, the absorber layer120 is fabricated from VLWIR material, and the barrier layer 130 isfabricated from (GaSb)_(x)(AlAs_(0.08)Sb_(0.92))_(1-x) lattice matchedto GaSb, with x value chosen such that the VBO of the first barrierlayer 131 is lined up with that of the absorber layer 120. The contactlayer 140 is fabricated from InAs_(0.9107)Sb_(0.0893). As shown by lines102 and 104 in FIG. 7, the VBO is above the CBO at the interface betweenthe second barrier layer 132 and the contact layer 140.

As compared to the embodiment depicted in FIG. 5, the y value is chosensuch that the bottom of the quantum well for holes at the tunnelinginterface is below the Fermi level, whereas in the example of FIG. 5,the bottom of the quantum well for holes at the tunneling interface isabove the Fermi level.

It should be understood that the examples provided by FIGS. 3-7 areprovided for illustrative purposes only, and that other materials andconfigurations are also possible to provide the tunneling structuresdepicted in FIGS. 1 and 2A-2C.

In addition to the above single color (i.e., wavelength) devicestructures, the TBIRD design structure is also applicable for dual bandapplications. One example dual-band infrared detector device 801incorporating a TBIRD structure is illustrated by the band diagram ofFIG. 8A. The illustrated dual-band infrared detector device 801 includesa first absorber layer 820 made of a material capable of absorbingradiation in a first wavelength range (e.g., LWIR), a barrier layer 830comprising a first barrier layer 831 and a second barrier layer 832, anda second absorber layer 840 made of a material capable of absorbingradiation in a second wavelength range (e.g., MWIR). It is noted thatthe barrier layer 830 may be configured as graded barrier layer asdescribed above rather than a first barrier layer 831 and a secondbarrier layer 832. The first absorber layer 820 may be fabricated from aLWIR material and the second absorber layer 840 may be fabricated from aMWIR material. The first and second barrier layers 831, 832 may befabricated from the materials of the first and second barrier layers131, 132 depicted in FIG. 5, for example.

The valence band of the dual-band infrared detector device 801 isillustrated by line 804, and the conduction band is illustrated by line802.

The tunneling structure 833 features a rectangular hole quantum welldesign. For illustrative purposes, it is assumed that the bias isdesignated to be positive if positive voltage is applied to the rightside of the device structure illustrated in FIG. 8A (i.e., a contact(not shown) proximate the second absorber layer 840). Under zero ornegative bias, the device simulated in FIG. 8A will have long-waveresponse. The long-wave response will be quenched after certain positivebias when the mid-wave-only response will be present, as illustrated inFIG. 8B.

Another application using the tunneling structure described above is afast tunneling barrier infrared detector (“Fast TBIRD”) device utilizingp-type absorber material. As the minority carrier in a p-type absorbermaterial is electron, the mobility is very high, and the carrierlifetime is also much shorter (thus “fast”) than that in an n-typematerial. Five non-limiting example Fast TBIRD device embodiments aredescribed and illustrated herein.

A first example Fast TBIRD device 900 is illustrated by a band diagramin FIG. 9. The example Fast TBIRD device 900 comprises an absorber layer920, a first barrier layer 930, which is graded, a n-type layer 950adjacent to the first barrier layer 930, a second barrier layer 935adjacent to the n-type layer 950, and a contact layer 940 adjacent tothe second barrier layer 935. The bandgap of the n-type layer 950 islarger than the bandgap of the absorber layer 920.

The absorber layer 920 and the first barrier layer 930 are doped p-type.The n-type layer 950 and the contact layer 940 are doped n-type.

The valence band of the Fast TBIRD device 900 is illustrated by line904, and the conduction band is illustrated by line 902.

When an infrared photon (IR) is absorbed in the smaller bandgap absorbermaterial of the absorber layer 920, an electron (E)-hole (H) pair isgenerated in the absorber layer 920. As the absorber layer 920 isp-type, electrons are minority carriers. The minority electron Epropagates toward the first contact layer (not shown), which in theillustrated embodiment is to the left of the page. The majority hole Hpropagates toward the first barrier layer 930. There will be anelectron-hole exchange at the tunneling junction provided by thetunneling structure 933 between the first barrier layer 930 and then-type layer 950. Due to the presence of the second barrier layer 935,the majority of electrons from the contact layer 940 cannot propagate tothe n-type layer 950 directly. Thermal generation or a shorterwavelength incident photon will enable generation of an electron-holepair within the n-type layer 950, as shown in FIG. 9. This provides amajority electron E to pass though the tunneling junction as well as aminority hole H to carry out the signal to the contact layer 940. Thedark current is dominated by the wide bandgap nBn device structure.

FIG. 10 depicts a simulation band diagram 1001 of an example embodimentof the Fast TBIRD structure illustrated in FIG. 9. In the simulation,the absorber layer 920 is fabricated from LWIR material that is dopedp−. The graded first barrier layer 930 is fabricated from(GaSb)_(x)(AlAs_(0.08)Sb_(0.92))_(1-x) with linearly graded x values,and is doped p-type to about 5×10¹⁶ cm⁻³. The n-type layer 950 and thecontact layer 940 on both sides of the second barrier layer 932 arefabricated from a MWIR material with a bandgap wider than the absorberthat is doped n-type in the MWIR regions to a doping level of about8×10¹⁶ cm⁻³. The second barrier layer 935 is fabricated from nominallyundoped (GaSb)_(y)(AlAs_(0.08)Sb_(0.92))_(1-y). As shown in FIG. 10, thegraded first barrier layer 930 and the n-type layer 950 provides atunneling structure 933 that creates a tunneling junction forelectron-hole exchange.

A second example Fast TBIRD device 1100 is illustrated by a band diagramin FIG. 11. In this embodiment, a TBIRD structure replaces the nBnstructure in the Fast TBIRD device 900 illustrated in FIG. 9.

The valence band of the Fast TBIRD device 1100 is illustrated by line1104, and the conduction band is illustrated by line 1102.

The example Fast TBIRD device 1100 comprises an absorber layer 1120, afirst barrier layer 1130, which is graded, an n-type layer 1150 adjacentto the first barrier layer 1130, a second barrier layer 1135 adjacent tothe n-type layer 1150, a third barrier layer 1132 adjacent to the secondbarrier layer 1135, and a contact layer 940 adjacent to the thirdbarrier layer 1132. The bandgap of the n-type layer 1150 is larger thanthe bandgap of the absorber layer 1120. The first barrier layer 1130 andthe n-type layer 1150 form a first tunneling structure 1133.

The materials of the absorber layer 1120, the first barrier layer 1130,the n-type layer 1150, the second barrier layer 1135, and the contactlayer 140 may be similar to the materials described above with referenceto FIGS. 9 and 10. As a non-limiting example, the third barrier layer1132 may be (GaSb)_(x)(AlAs_(0.08)Sb_(0.92))_(1-x) with x value chosensuch that there is the broken gap type II band alignment with thecontact layer 1140 MWIR material. The third barrier layer 1132 and thesecond contact layer 140 form a second tunneling structure 1137.

FIG. 12 provides a simulation band diagram 1201 of the Fast TBIRD device1100 depicted in FIG. 9 using the same materials for the various layersas described above with respect to the simulation of FIG. 10. In thesimulation depicted in FIG. 12, the third barrier layer 1132 isfabricated from (GaSb)_(x)(AlAs_(0.08)Sb_(0.92))_(1-x) that is dopedp-type to 2×10¹⁷ cm⁻³.

A third example Fast TBIRD device 1300 is illustrated by a band diagramin FIG. 13. In this embodiment, the Fast TBIRD device 1300 comprises anabsorber layer 1320, a hole barrier layer 1360 providing a p-n junction1362, a first barrier layer 1332, a second barrier layer 1330, and acontact layer 1340.

The valence band of the Fast TBIRD device 1300 is illustrated by line1304, and the conduction band is illustrated by line 1302.

A tunneling structure 1333 is provided by the hole barrier layer 1360and the first barrier layer 1332.

After one electron E and hole H pair is generated in the p-type absorberlayer 1320, the minority electron E will drift toward the wider gap p-njunction within the hole barrier layer 1360, and move down to the lowestavailable energy states within the electron well formed before thetunneling structure 1333, where the minority electron E will meet andrecombine with a hole H from the hole well on the barrier layer 1330side of the tunneling structure 1333. Charge neutrality requires oneextra hole H to flow in from the contact layer 1340 side of the devicestructure and thus the signal is detected in an external circuit. Thedevice dark current is limited by either the generation-recombination(“GR”) current from the wider-gap p-n junction within the hole barrierlayer 1360, the diffusion current from the absorber layer 1320, or thediffusion current from the contact layer 1340.

FIG. 14 provides a simulation band diagram 1401 of the Fast TBIRD device1300 depicted in FIG. 13. The material of the absorber layer 1320 is adoped p− MWIR material. The hole barrier layer 1360 providing the holebarrier of the non-limiting example embodiment is an material with awider bandgap than that of the absorber layer 1320. In the illustratedsimulation, the p and n doping levels within hole barrier layer 1360forming the p-n junction is in the mid 10¹⁵ cm⁻³ range. The firstbarrier layer 1332 is formed using(GaSb)_(x)(AlAs_(0.08)Sb_(0.92))_(1-x) p doped to 3×10¹⁸ cm⁻³. Thefollowing second barrier layer 1330 is formed using(GaSb)_(y)(AlAs_(0.08)Sb_(0.92))_(1-y) with y>x and a background p-typedoping on the order of 10¹⁵ cm⁻³. The contact layer 1340 is formed usingthe same MWIR material as the absorber layer 1320 with n-type dopingchosen to create close-to-flat band alignment for holes with the barrierlayer 1330. A tunneling junction is present at the interface between thehole barrier layer 1360 and the barrier layer 1332 such that the VBO ofthe hole barrier layer 1360 is above the CBO of the first barrier layer1332.

A fourth example Fast TBIRD device is illustrated by a simulation banddiagram 1501 in FIG. 15. The Fast TBIRD device illustrated in FIG. 15 isa derivative of the Fast TBIRD device 1300 illustrated in FIG. 13. Thefirst barrier layer 1332′, the second barrier layer 1330′, and thecontact layer 1340′ are p+ doped. More specifically, the first barrierlayer 1332′ is Al_(0.25)Ga_(0.75)As_(0.02)Sb_(0.98) doped p+, thebarrier layer 1330′ is AlAs_(0.08)Sb_(0.92) doped p+. In the illustratedexample, the absorber layer 1320′ is fabricated from LWIR material, thehole barrier layer 1360′ is fabricated from MWIR material, and thecontact layer 1340′ is fabricated from a same material as layer 1360except for the p doping.

In this example, there are no true “barriers” except for a spike in thevalence band at the interface between the first barrier layer 1332′ andthe second barrier layer 1330′. At the interior of the device structure,the holes flow through the barrier with little resistance. On thesidewall surface, the Aluminum containing layers will oxidize, and blockthe current flow. Despite that fact, that the absorber layer 1320 couldbe inverted to be n-type on the surface, the associated shunt path isblocked by the oxidized Al containing layers. Thus this device structurecould resolve the passivation issue for p-type absorber materials.

A fifth example Fast TBIRD device is illustrated a simulation banddiagram 1601 in FIG. 16. This embodiment is a simplification of thedevice structure illustrated in FIG. 15. The device comprises anabsorber layer 1620, a hole barrier layer 1660, an n++ layer 1638 with asame bandgap as hole barrier layer 1660, a barrier layer 1630, and acontact layer 1640. The first barrier layer 1332 of FIG. 15 iseliminated. Carriers would propagate through the structure viaband-to-band tunneling. In order to facilitate the band-to-bandtunneling, a thin (e.g., a few to tens of nanometers thick) n++ layer1638 with a same bandgap of the hole barrier layer 1660 is doped n++adjacent to the barrier layer 1630. The hole barrier layer 1660 providesa p-n junction 1662. In the illustrated embodiment, the absorber layer1620 is fabricated from LWIR material, the hole barrier layer 1660 isfabricated from MWIR material, the n++ layer 1638 is fabricated from asame material with hole barrier layer 1660 doped n++, the barrier layer1630 is fabricated from AlAs_(0.08)Sb₀₉₂ doped p++. and the contactlayer 1640 is fabricated from a same material as hole barrier layer 1660doped p+.

Compared with a pBp device design, the example Fast TBIRD devicesdescribed above disclosure would alleviate or completely resolve thesurface passivation issues. It should be understood that the embodimentsof a Fast TBIRD device are not limited to the examples given in thisdocument. The structure can be modified easily to meet the needs for anyspecific application.

The foregoing description of the various embodiments of the presentdisclosure has been presented for the purposes of illustration anddescription. Many alternatives, modifications and variations will beapparent to those skilled in the art of the above teaching. Moreover,although multiple aspects have been presented, such aspects need not beutilized in combination, and various combinations of aspects arepossible in light of the various embodiments provided above.Accordingly, the above description is intended to embrace all possiblealternatives, modifications, combinations, and variations that have beendiscussed or suggested herein, as well as all others that fall with theprinciples, spirit and broad scope of the subject matter as defined bythe claims.

What is claimed is:
 1. An infrared detector device comprising: a firstcontact layer; an absorber layer adjacent to the first contact layer;and a tunneling structure comprising a barrier layer adjacent to theabsorber layer and a second contact layer adjacent to the barrier layer,wherein the barrier layer has a tailored valence band offset such that:a valence band offset of the barrier layer at the interface between theabsorber layer and the barrier layer is substantially aligned with thevalence band offset of the absorber layer; and the valence band offsetof the barrier layer at the interface between the barrier layer and thesecond contact layer is above a conduction band offset that is withinthe second contact layer.
 2. The infrared detector device of claim 1,wherein: the barrier layer comprises a first barrier layer and a secondbarrier layer; the first barrier layer is adjacent to the absorberlayer; and the second barrier layer is adjacent to the second contactlayer.
 3. The infrared detector device of claim 2, wherein: the firstbarrier layer comprises (GaSb)_(x)(AlAs_(0.08)Sb_(0.92))_(1-x) latticematched to GaSb, with x value chosen such that a valence band offset ofthe first barrier layer is substantially-aligned with a valence bandoffset of the absorber layer; and the second barrier layer comprises(GaSb)_(y)(AlAs_(0.08)Sb_(0.92))_(1-y), where y>x.
 4. The infrareddetector device of claim 3, wherein a base energy level of a quantumwell for holes formed by the tunneling structure is greater than theFermi level.
 5. The infrared detector device of claim 3, wherein a baseenergy level of a quantum well for holes formed by the tunnelingstructure is less than the Fermi level.
 6. The infrared detector deviceof claim 1, wherein the tunneling structure provides a gradualtransition from the valence band offset at the interface between thebarrier layer and the absorber layer to the valence band offset at theinterface between the barrier layer and the second contact layer.
 7. Theinfrared detector device of claim 1, wherein a transition from thevalence band offset at the interface between the barrier layer and theabsorber layer to the valence band offset at the interface between thebarrier layer and the second contact layer is parabolic.
 8. The infrareddetector device of claim 1, wherein a transition from the valence bandoffset at the interface between the barrier layer and the absorber layerto the valence band offset at the interface between the barrier layerand the second contact layer is rectangular.
 9. The infrared detectordevice of claim 1, wherein the absorber layer comprises an InAs/InAsSbSLS, InAs/Ga(In)Sb SLS, a digital alloy, or bulk absorber.
 10. Theinfrared detector device of claim 1, wherein the barrier layer comprisesa graded (GaSb)_(x)(AlAsSb)_(1-x) structure.
 11. The infrared detectordevice of claim 1, wherein the barrier layer comprises a gradedsuperlattice material.
 12. The infrared detector device of claim 11,wherein the graded superlattice material comprises a AlAsSb/GaSbsuperlattice structure.